1. Field of the Invention
Embodiments of the invention relate to methods of making titania nanostructures and more particularly to electrochemical methods of making titania nanostructures.
2. Technical Background
Metal oxides are material systems explored, in part, due to metal oxides having several practical and industrial applications. For example, titanium (IV) oxide (titania) is used in a wide range of applications such as in paints, cosmetics, catalysis, and bio-implants.
Nanomaterials possess unique properties that are not observed in the bulk material, for example, the optical, mechanical, biochemical and catalytic properties of particles are closely related to the size of the particles. In addition to very high surface area-to-volume ratios, nanomaterials exhibit quantum-mechanical effects which can enable applications that are otherwise impossible using the bulk material. One of the challenges with nanotechnology is the manufacture of nanomaterials in an economically viable process. As a result, only a very few nanotechnology based applications have been commercialized, although a wide spectrum of nanotechnology based applications have been demonstrated on a laboratory scale.
Titania, for example, is a material system where nanotechnology based applications have been demonstrated on a laboratory scale and where the nanomaterials could be used in a wide range of practical applications. Titania nanomaterials can be used, for example, in photovoltaic applications such as dye-sensitized solar cells, metal-semiconductor Junction Schottky Diode solar cells, and doped-TiO2 nanomaterials based solar cells. Titania nanomaterials can be used in photocatalysis, photo-degradation of various organic pollutants, for example, Rhodamine B, Chloroform, Acid Orange II, Phenol, Salicylic Acid, and Chlorophenols. Further, titania nanomaterials are useful in hydrogenation reactions, for example, hydrogenation of propyne (CH3CCH), photocatalytic water splitting. Also, titania nanoparticles can be used in electrochromic devices such as electrochromic windows and displays, in hydrogen storage, in sensing applications, for example, humidity sensing and gas sensing such as in hydrogen, oxygen, carbon monoxide, methanol, and ethanol sensors. Titania nanomaterials can be used in lithium batteries as insertion electrodes.
There are several conventional methods for the synthesis of titania nanomaterials, for example, sol-gel, micelle and inverse micelle, sol, hydrothermal, solvothermal, direct oxidation, chemical vapor deposition, physical vapor deposition, electrodeposition, sonochemical, microwave, organic templated synthesis, aerogel, and TiO2 nanosheets, for example, through delaminated layer synthesis from protonic titanate.
In conventional sol-gel methods, a colloidal suspension or sol is formed from precursors, typically inorganic metal salts or metal-organic compounds, for example, metal alkoxides through hydrolysis and polymerization reactions. Loss of solvent and complete polymerization leads to the transition into a sol-gel phase which is then converted into a dense ceramic through further drying and heat treatment. Typical synthesis of titanium oxide nanomaterials using the sol-gel method includes adding titanium alkoxide (e.g. titanium tetraisopropoxide) precursor to a base such as tetramethyl ammonium hydroxide at 2° C. in alcoholic solvents. This is followed by heating at from 50° C. to 60° C. for 13 days or at from 90° C. to 100° C. for 6 hours and finally subjecting to a secondary treatment involving heating in an autoclave or high-pressure reactor at from 175° C. to 200° C.
Conventional sol-gel methods employ extreme process conditions, for example very low temperature to high temperatures and pressures with high energy requirements, requires high pressure reactors with increased capital costs and uses chemicals, for example, isopropoxides that involve increased handling costs.
In conventional hydrothermal methods, hydrothermal synthesis is performed in an autoclave or high pressure reactor with Teflon 4 liners under controlled temperature and pressure with the reactions occurring in aqueous solutions.
A variation of this method is the solvothermal method wherein organic solvents are used instead of an aqueous environment. Typical synthesis of titanium oxide nanowires involves reacting titanium chloride with an acid or inorganic salt at from 50° C. to 150° C. in an autoclave for 12 hours. This is followed by washing powders of nanomaterial in DI water and ethanol and drying at 60° C. for several hours.
Some of the other conventional hydrothermal methods for making titania nanoparticles are hydrothermal reaction of titanium butoxide (in isopropanol) with water (water:Ti ratio of 150:1) at 70° C. for 1 hour followed by filtration and heat treatment at 240° C. for 2 hours and finally washing in DI water and/or ethanol and drying at 60° C.; hydrothermal reaction of titanium alkoxide precursor in acidic ethanol-water solution at 240° C. for 4 hrs followed by washing and drying; and a method of making TiO2 nanowires through a hydrothermal treatment of TiO2 powder in from 10 molar to 15 molar sodium hydroxide at from 150° C. to 200° C. for from 24 hours to 72 hours followed by washing and drying.
Conventional hydrothermal methods have disadvantages similar to the sol-gel method, for example, high cost autoclaves, use of chemicals that require careful handling, in addition to being time-consuming and having expensive post-processing treatments.
In conventional electrodeposition methods, titania nanowires are deposited using an anodic alumina membrane (AAM) as template. The synthesis is carried out in a titanium chloride solution (at pH=2) using pulsed electrodeposition. The substrate is subsequently heated to 500° C. for 4 hours followed by removal of the AAM template. A prerequisite for this method is the availability of a template that can be removed without leaving any residue using a moderate removal process. Otherwise, regular electrodeposition yields bulk sized particles. Additionally, handling of corrosive electrolyte like titanium chloride in an industrial process can be challenging.
In conventional direct oxidation methods, synthesis of titania nanotubes involves applying a voltage of from 10 volts to 20 volts for from 10 minutes to 30 minutes between two titanium plates in a 0.5% hydrogen fluoride (HF) solution. The use of HF makes this process unattractive for industrial production. Also, the shape of the nanostructures obtained is limited to nanotubes.
Conventional methods of making titania nanostructures are energy intensive, employ expensive capital equipment, for example, high pressure reactors, involve tedious process steps, for example, cleaning, washing and drying of powders, and use nonbenign chemicals, for example, alkoxides, titanium chloride, and HF.
It would be advantageous to have method of making titania nanomaterials in large quantities in an economically viable fashion.